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Alloys size affect speedo?



  172 cup
Hi i have a 1.2 16v clio 02 plate and was wondering if i put larger alloys on my car having the 14" standard alloys at the moment will this affect the way the speedo works by giving false readings
 

DrR

ClioSport Club Member
  VW Golf GTD
Work out the radius of the wheels, 16" with low profiles tyre could be same as 14" with bigger tyres
 
  172 Cup, AX trackbeast.
shouldnt do as most cars' speedo's are actually run off the gearbox not the wheels. :)


Err, the same gearbox that runs the wheels?!

It's all to do with rolling radius. You need to pay attention to the diameter of the wheel and the aspect ratio of the tyre. (The small number after the width of the tyre)

As an example. Say you have 195/60/14s. That means the tyre is 195mm wide, with the sidewall being 60% of the width, running on a 14 inch rim. Usually, if you go up 1 inch on wheel diameter, you go up 10mm on with and come down 5% on the aspect ratio. So, you'd end up with 205/55/15 or 215/50/16 and so on.
 

Ian

  Focus TDCi
14/175/65 (Si 'Silvia' Wheels) and 16/195/45 (172 alloys) give:

29.095cm
29.08cm

rolling radius, respectively. Think that's right.. it's the change I made anyway.
 

DMS

  A thirsty 172
Using old school mathematics, that should make the circumference of the tyres about 1.5 to 2 inches more at most.
So they'll go from being about 53 to about 55 inches in circumference. (I think, but I have been drinking!)
So I wouldn't worry too much about speeding. I think most cars' speedo's under-indicate the speed you're doing anyway, so as long as you're sat right on the speed limit or less when passing a camera / sleeping policeman you shouldn't have any issues.
If you're that concerned about the speed you could always get a sat nav that indicates your speed measured by GPS. I think that's a bit more accurate than your car's speedo, although it's still not 100% perfect.
 

Struggler

ClioSport Club Member
  Ph1 track 172
i have 17, does affect the speedo, but by like 3-5%,

really wouldnt loose sleep over it, i still go thru cameras with my speedo on 30mph lol
(where as im probably doing 31!!!!)
 

Ian

  Focus TDCi
If you stick to the sizes I've said your speedo will be out by 0.07% at 70mph.

14s - 70mph
16s - 70.05mph.

Again, assuming my maths is correct.
 


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